The subject invention concerns a device for use in power-driven hand tools to hold and control the hand tool during operation thereof.
One example of a hand tool which is contemplated to be fitted with the device in accordance with the subject invention is the power saw or chain saw which is used in forest management e.g. for felling and for lopping off branches. In lopping operations which involve sawing off branches from felled, recumbent tree trunks it is necessary to direct the guide bar of the chain saw in such a manner that its plane is parallel with or extends along the trunk of the tree to ensure that minimum amounts of the knots or branches remain after the cutting operation. To ensure this guide bar position at all times the operator need either to shift his grip on the handles or twist his hands and/or arms in such a manner that he obtains the desired angle with respect to each individual knot or branch. If the operator choses the first possibility to work with the chain saw, he must perform a complicated manoeuvre which, if it is to be performed safely, requires that the operator puts down the saw on a support before he shifts his grip thereon. To alter the angle of hands and arms, the solution most commonly adopted for reasons of convenience, is extremely unsuitable because it prevents the operator from exercising adequate muscular strength and therefore lessens his ability to control the saw. This could have serious consequences, such as bodily injuries, in case the saw jams or jerks. From an ergonometrical viewpoint it is also unsuitable to work with the chain saw if the operator has to hold his hands and/or arms in abnormal positions, which in addition to giving the disadvantages mentioned above results in uncomfortable and tiring working positions which in turn may lead to work injuries, such as backaches and worn-out joints.
A chain shaw construction intended to make it possible to operate the chain saw without having to assume the hitherto necessary uncomfortable and unsuitable work positions is suggested in the Swedish Patent Application No. 8001841-9 (corresponding to DE-OS-No. 2 909 777). This publication describes a chain saw the front bow handle of which is pivotably mounted for movement about the longitudinal axis of the saw body and which comprises a mechanism by means of which the handle may be blocked in a number of different angular position. The rear grip handle together with the throttle control is, however, fixedly mounted to the body of the saw.
This construction does not constitute a significant improvement in safety since the saw operator still needs either to change his grip on the rear saw handle or else bend his wrist into an abnormal position. This prior-art construction also increases the load on the wrist since it is necessary, when pivoting the bow handle downwards, both to counteract the movement of the saw and to carry most of the weight of the saw by the hand holding the rear handle. Instead of reducing the risks of work injuries this constructions tends to increase the possible hazards. This is confirmed in research investigations carried out by the Swedish National Board of Industrial Safety. This research shows that the right hand, i.e. precisely the hand which normally manipulates the rear saw handle, is the one most frequently injured. This is essentially due to the very considerable number of bending movements of the load-carrying wrist that the right hand has to perform. The number of such bending movements evidently is increased considerably when using a chain saw constructed as taught in the Swedish Patent Application No. 8001841-9 referred to above, and consequently this teaching cannot be regarded to offer a satisfactory solution to the problems involved.
The purpose of the subject invention is to provide a power-driven hand tool in which the problems outlined in the aforegoing have been eliminated.